1. Field of Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and, in particular, to dynamic random access memory (DRAM) having a capacitor with properties of low leakage current and high permittivity.
2. Description of Related Arts
DRAM has been used for a semiconductor memory operable at a high speed in a computer or other electronic devices. DRAM has a memory cell array and a peripheral circuit for operating the array. The memory cell array has a plurality of units arranged in a matrix, and each unit comprises one switching transistor and one capacitor.
As in other semiconductor devices, DRAM has developed with miniaturization of each cell to satisfy a demand for high-integration. As a result, the area on which a capacitor is formed decreases, and it is thus difficult to ensure the capacity required for a memory device. To solve this problem, a three-dimensional structure of electrodes, upper and lower electrodes made of a metallic material (MIM structure), a capacitive insulation film having high permittivity, etc have been introduced. Currently, DRAM with a minimum feature size (F value) of 70 nm or less, which is used as a standard index of a technology level, necessarily has a three-dimensional electrode structure, and upper and lower electrodes made of a metallic material have already been used in practice. Therefore, the prospect of improving the features of a capacitor on the basis of these technical developments is bleak. The current trend of additional miniaturization mainly includes improving the feature of a capacitor by high permittivity of a capacitive insulation film left for the last.
Recently, a capacitor with a MIM structure, e.g., a structure of TiN/ZrO2/TiN, has been used as a DRAM capacitor.
DRAM is formed from heat treatment at 450° C. to 500° C. as an unavoidable process after a capacitor has been formed. However, a dielectric film made of a single body of zirconium oxide cannot achieve a sufficient thermostability, and leakage current likely increases after heat treatment.
Therefore, several attempts have been made to increase thermostability, and the examples of such attempts include a multilayer dielectric film, e.g., a ZAZ structure where Z and A mean ZrO2 and Al2O3, respectively, in ZrO2/Al2O3/ZrO2, or a laminating layer having a plurality of Al2O3 and ZrO2 layers alternately.
This structure aims to accomplish the desirable characteristic from the combination of zirconium oxide (ZrO2) having high permittivity and aluminum oxide (Al2O3) having high thermostability instead of high permittivity.
For example, JP 2006-135339 A discloses an AZ, ZA, or ZAZ structure, or a method for forming a multilayer dielectric film alternately laminating a ZrO2 thin film and an Al2O3 thin film.
In practical DRAM, a heat treatment with a process of forming an upper electrode, which is to be performed after forming a dielectric film of a capacitor, or a heat treatment such as hydrogen annealing for reducing the interface state of a transistor, is carried out. However, this heat treatment, which is performed after forming a dielectric film comprising zirconium oxide such as a ZAZ structure as a principal component, may cause leakage current of a capacitor. This result may impose restrictions on the manufacturing process, and make it difficult to achieve a good characteristic of transistors by a sufficient hydrogen annealing. As described herein, a dielectric film having zirconium oxide as a primary constituent means a dielectric film having a composite ratio of 0.8 or more, the composite ratio being indicated Z/(Z+M) where Z is the number of zirconium atoms and M is the number of metallic atoms other than zirconium in the dielectric substance.
The inventors in this invention found out that the increase in leakage current in a capacitor, which has been caused with the heat treatment, may occur due to the combination of (1) the degradation of the dielectric film itself by the increase of oxygen deficit in a dielectric film or by the diffusion of impurities such as nitrogen, (2) the partial desquamation of a dielectric film, which has zirconium oxide as a primary constituent, from titanium nitride electrodes, and (3) damages, such as crack, on a dielectric film itself. These events are considered to stem from a change in stress caused particularly by the heat treatment under reducing atmosphere or by the secondary growth of crystal grains of a dielectric film.